Various processes have been studied in the past as powder metallurgy methods for titanium alloys, and many of these have been subjected to practical application. These processes are categorized by the type of powder raw materials used, and by the type of forming process used in obtaining a compact from these raw materials. Specifically, depending on the powder raw materials used, these processes are broadly grouped into blended elemental method and prealloyed method, and depending on the forming process employed, these processes are classified into die forming method, hydrostatic pressure forming method, injection molding method and so on.
First, of the processes broadly grouped by the powder raw materials used, the blended elemental method involves the use of a mixed powder obtained by mixing specific proportions of pure titanium powder and a metal powder used for adding the alloy element, namely a powder of the metal that is added as the alloy element to the pure titanium powder, as the powder raw material. The prealloyed method, which is the other type of method, involves the use of an alloy powder that has already been alloyed as the powder raw material.
Meanwhile, of the classifications by the forming process used in obtaining a compact from the powder raw materials, a die forming method involves obtaining the targeted compact by injecting the above-mentioned mixed powder raw material or alloy powder raw material into a specific die, with a small amount of lubricating oil added, and then press forming the contents. In a hydrostatic pressure forming method, a mold that is sufficiently flexuous, composed of rubber, plastic or the like, is used as the mold, the inside of which is packed with the powder raw material, after which the inside of the mold is vacuum evacuated and sealed in this state, and then the sealed mold is subjected to hydrostatic pressure to form a compact. Finally, an injection molding method is a process in which a mixture obtained by mixing a powder raw material with a large quantity of binder and a lubricating oil or the like is extruded inside a specific mold, and then heated to decompose and remove the binder, which yields a compact.
A compact obtained by one of the above forming methods from a mixed powder raw material or alloy powder raw material is sintered by being further heated in a vacuum or in an inert atmosphere, and in a blended elemental method, the various elemental powder components contained in the mixed powder raw material are diffused and alloyed, thus yielding a targeted sintered body of the titanium alloy.
These various powder metallurgy methods for titanium alloys each have their own distinctive advantages, as will be discussed below, but they also have drawbacks. For instance, advantages to a blended elemental method are that because relatively soft pure titanium is used, the forming is easy, and the material cost is lower than with a prealloyed method in which an expensive alloy powder is used, but a drawback is that in the event that the mutual diffusion coefficient of titanium is different from that of the metal used for the alloy element, the Kirkendall effect causes the sintered body to be prone to cavity formation in the sintering and alloying process. Also, in the event that the melting points of the titanium and the metal used for the alloy element are markedly different, first of all, when the metal with the lower melting point is melted, the compact will expand and cracks will develop in it, or the molten metal will infiltrate the grain boundary and form cavities, or this molten metal will react with the titanium to form a brittle intermetallic compound. As a result, there is the problem of a marked decrease in the strength of the sintered body obtained.
In a die forming method, an advantage is a lower processing cost than in other forming methods, but on the other hand, because the pressing during forming is only performed in one direction (the press direction of the die), in order for a uniform pressure to act on the entire powder raw material, the particles that make up the powder raw material must be capable of moving easily as a result of the local pressure differential within the die, but in actual practice, friction between the die and the powder raw material or between the particles of the powder raw material precludes sufficient movement of the particles of the powder raw material during press forming, and as a consequence, particularly when the targeted compact has a complex shape, it is impossible to apply the pressure uniformly to the powder raw material, and in turn to the compact. Therefore, in the case of a compact with a complex shape, excessive shear stress can occur locally, and this can cause shear cracking to occur. Also, it is geometrically advantageous to use a powder raw material with a finer particle size in order to raise the density of the compact with a die forming method, but the finer is the particle size of the powder raw material, the greater is the above-mentioned friction between the particles of the powder raw material and so on, so there is a limit to how much the density of the compact, and in turn that of the sintered body obtained from this compact, can be increased with a die forming method.
Furthermore, with a hydrostatic pressure forming method, because a sufficiently flexuous mold composed of rubber, plastic or the like is used, and because the hydrostatic pressure is applied to this mold in a state in which the mold has been pressed tightly against the powder raw material by vacuum suction, the pressure can be applied almost evenly to the entire surface of the mold that surrounds the powder raw material even if the targeted compact has a complex shape, so a compact with a complex shape can be obtained. On the other hand, with this method, processes for preparing the mold, the vacuum suction, the tight pressing of the mold and the application of the hydrostatic pressure and so on are required, which causes a problem in that it makes the forming process more complicated and drives up the processing costs.
With an injection molding method, a mixed powder raw material or an alloy powder raw material is used in a state in which it is mixed with a large quantity of binder and a lubricating oil or the like, so this powder raw material has high fluidity, and sufficient movement can be obtained for the particles of the powder raw material during press molding, which means that the pressure can be applied almost evenly to the powder raw material, and in turn to the compact, even when the targeted compact has a complex shape. As a result, a feature of this method is that a compact with a complex shape can be obtained, but the down side to an injection molding method is that the above-mentioned binder removal step takes about half a day at a temperature of several hundred degrees, and an expensive powder raw material having a particle size of no more than a few dozen microns is required, so processing costs and raw material costs are disadvantages. Also, since this binder removal is completed in a state in which a small amount of binder is left in the compact in order to preserve the shape of said compact in the sintering step, with a highly active metal such as titanium, this binder forms carbides and oxides with the titanium in the sintering step, and this can result in a loss of strength and other such characteristics of the sintered body obtained. Furthermore, it is difficult to remove the binder uniformly if the depth of the targeted compact are thick, and even if the binder is removed under ideal conditions for the surface layer of said compact, a large amount of binder will still be left behind in the interior thereof, which leads to the above-mentioned deterioration in characteristics and to serious deformation during sintering. As a result, the size of the compact to which an injection molding method can be applied is limited to compacts weighing a few dozen grams at most.